This invention relates to radio frequency (RF) devices and more particularly to such devices operating in a vacuum-like environment.
During recent years the electronics industry has shown considerable interest in eliminating RF voltage breakdown occurrence in RF and microwave components designed for operation in space or a similar vacuum-like environment.
For spacecraft components such as filters, diplexers, switches and antennas, voltage breakdown results in a loss of transmission. It reduces efficiency of the component by an increase of insertion loss, impedance mismatch or by de-tuning, and more severely a permanent damage to the component may result by local heating effects.
The RF voltage breakdown in the presence of moderate to high RF power levels and in pressures lower than 10.sup.-2 millimeters of mercury is caused by the secondary electron emission from RF electrodes of the RF device. The RF electrodes are defined as the center conductor of a TEM resonator and cavity wall (or tuning disc) or terminals of an RF switch or antenna. Under condition that the electron emission coefficient of electrodes is greater than unity (this includes most conductors and insulators) the high energy primary electrons upon impact on a solid surface release a greater number of secondary electrons. If the mean free path of electrons is longer than the electrode separation d the secondary electrons are accelerated by multiple half-cycles of the RF field and obtain the energy levels of the primary electrons. This process repeats itself until the multiple impacting or multipacting of electrons constitutes a space charge saturation. The space charge saturation lowers RF high voltage gap inpedance and gives rise to RF voltage breakdown.
In the past to avoid multipacting the component manufacturers have pressurized the cavities of microwave components with suitable inert gas. Component pressurization requires a pressure vessel, or the components are designed with heavier housing to withstand expected pressure levels usually greater than 760 millimeters of mercury in high vacuum. The pressurization arrangement adds volume and weight and reduces the reliability of the design.